Researchers: Katrina Weakened Forests' Ability to Store Carbon

Nov 16, 2007

With
the help of National Aeronautics and Space Administration (NASA)
satellite data, a research team has estimated that Hurricane Katrina
killed or severely damaged 320 million large trees in Gulf Coast
forests, which weakened the role the forests play in storing carbon
from the atmosphere. The damage has led to these forests releasing
large quantities of carbon dioxide to the atmosphere.

The August 2005 hurricane affected five million acres of forest
across Mississippi, Louisiana and Alabama, with damage ranging from
downed trees, snapped trunks and broken limbs to stripped leaves.

Young growing forests play a vital role in removing carbon dioxide,
a greenhouse gas, from the atmosphere by photosynthesis, and are thus
important in slowing a warming climate, NASA's Goddard Space Flight
Center stated. An event that kills a great number of trees can
temporarily reduce photosynthesis, the process by which carbon is
stored in plants. More importantly, all the dead wood will be consumed
by decomposers, resulting in a large carbon dioxide release to the
atmosphere as the ecosystem exhales it as forest waste product. The
team's findings were published Nov. 15 in the journal Science.

"The loss of so many trees will cause these forests to be a net
source of carbon dioxide to the atmosphere for years to come," said the
study's lead author Jeffrey Chambers, a biologist at Tulane University
in New Orleans. "If, as many believe, a warming climate causes a rise
in the intensity of extreme events like Hurricane Katrina, we're likely
to see an increase in tree mortality, resulting in an elevated release
of carbon by impacted forest ecosystems."

Young forests are valued as carbon sinks, which remove carbon
dioxide from the atmosphere and store it in growing vegetation and
soils. In the aftermath of a storm as intense as Katrina, vegetation
killed by the storm decomposes over time, reversing the carbon storage
process, making the forest a carbon source.

"The carbon cycle is intimately linked to just about everything we
do, from energy use to food and timber production and consumption,"
Chambers said. "As more and more carbon is released to the atmosphere
by human activities, the climate warms, triggering an intensification
of the global water cycle that produces more powerful storms, leading
to destruction of more trees, which then act to amplify climate
warming."

Chambers and colleagues from the University of New Hampshire in
Durham, N.H., studied Landsat 5 satellite data captured before and
after Hurricane Katrina to pull together a reliable field sampling of
tree deaths across the entire range of forests affected by Katrina.
They found that some forests were heavily damaged while others like the
cypress-tupelo swamp forests fared remarkably well.

The NASA-built Landsat 5, part of the Landsat series of
Earth-observing satellites, takes detailed images of the Earth's
surface. Chambers combined results from the Landsat image sampling with
data from the Moderate Resolution Imaging Spectroradiometer instrument
on NASA's Terra satellite to estimate the size of the entire forested
area affected by Katrina. The instrument can detect minute changes in
the color spectrum on the land below, enabling it to measure
differences in the percentage of live and dead vegetation. This helps
researchers improve their estimates of changes in carbon storage and
improves their ability to track the location of carbon sinks and
sources.

The field samples and satellite images, along with results from
computer models that simulate the kind of vegetation and other traits
that make up the forests, were used to measure the total tree loss the
hurricane inflicted. The scientists then calculated total carbon losses
to be equivalent to 60 percent to 100 percent of the net annual carbon
sink in U.S. forest trees.

"It is surprising to learn that one extreme event can release nearly
as much carbon to the atmosphere as all U.S. forests can store in an
average year," said Diane Wickland, manager of the Terrestrial Ecology
Program at NASA Headquarters in Washington. "Satellite data enabled
Chambers' research team to pin down the extent of tree damage so that
we now know how these kinds of severe storms affect the carbon cycle
and our atmosphere. Satellite technology has really proven its worth in
helping researchers like Chambers assess important changes in our
planet's carbon cycle."